A gas turbine combustor mixes large quantities of fuel and compressed air, and burns the resulting air and fuel mixture. Conventional combustors for industrial gas turbines typically include an annular array of cylindrical combustion “cans” in which air and fuel are mixed and combustion occurs. Compressed air from an axial compressor flows into the combustor. Fuel is injected through fuel nozzle assemblies that extend into each can. The mixture of fuel and air burns in a combustion chamber of each can. The combustion gases discharge from each can into a duct that leads to the turbine.
Pressurized air from the compressor enters a combustion can at the back end of the can, which is the same end from which hot combustion gases flow from the can to the turbine. The compressed air flows through an annular duct formed between a cylindrical wall of the can and an inner cylindrical combustion liner. The relatively cool compressed air cools the wall of the liner as the hot combustion gas flows through the interior of the liner. The hot combustion gas flows in a generally opposite direction to the flow of the compressed air through the duct.
As the compressed air reaches the head-end of the combustor can, the air is turned 180 degrees to enter one of the fuel nozzles. To enter the outer fuel nozzles the compressor air makes a tight and quick reversal of flow direction. This abrupt turn can create low velocity flow zones in the air while other zones of the air flow are at significantly higher velocities. The occurrence of low velocity flows is most acute as the air enters the outer fuel nozzles which are closest to the double walled flow path in the combustion chamber for compressed air.
Uniform flow velocities through a fuel nozzle are desired to provide uniform mixing of the air and fuel, and uniform combustion. Zones of low velocity airflow in the fuel nozzle also pose a flame holding risk inside the nozzle as low velocity zones provide an area for a flame to anchor inside the fuel nozzle. A flame in the fuel nozzle can destroy the hardware of the nozzle. In addition, low velocity air flows can cause localized variations in the air and fuel mixture. These variations can include regions where the fuel and air mixture is too rich resulting in too high combustion temperatures and excessive generation of nitrous oxides (NOx). There is a long felt desire to hold a steady flame in a combustor can, reduce NOx emissions from combustion in a gas turbine and maintain uniform airflow velocities through the fuel nozzles.